Vehicle communication systems locking and unlocking vehicle doors without using mechanical keys are practically used. More specifically, a keyless entry system locking and unlocking vehicle doors through a remote operation from a portable device held by a user, a smart entry (registered trademark) system unlocking vehicle doors when a user holding a portable device approaches a vehicle or is in contact with only a door handle, and the like are practically used.
In addition, a vehicle communication system enabling an engine or a driving battery system of a vehicle to be started without using a mechanical key (ignition key) is practically used as well. More specifically, a system called a push start system or the like in which an engine or a driving battery system is started by a user, who holds a portable device, only pushing a start button is practically used. In such a push start system, an in-vehicle device configuring a vehicle communication system performs a vehicle cabin inside or outside determining process determining whether or not an authorized portable device corresponding to a vehicle is located inside a vehicle cabin. In the push start system, only in a case where an authorized portable device is determined to be located inside the vehicle cabin, the engine or the driving battery system is started, thereby improving the convenience and the safety.
In the vehicle cabin inside or outside determining process, a position estimating technology using transmission and reception of radio signals between the in-vehicle device and the portable device is used. The position estimating technology using transmission and reception of radio signals is largely divided into a range-based system and a range-free system. The range-based system is a method in which, in the vehicle cabin inside or outside determining process, special information relating to a radio signal that is transmitted and received between a plurality of on-vehicle antennas disposed at different positions in a vehicle and a portable device, for example, a received signal strength indication (RSSI), a time of arrival (TOA), a time difference of arrival (TDOA), an angle of arrival (AOA), or the like of a radio signal is measured by the in-vehicle device or the portable device, and the position of the portable device is estimated based on a difference between measurement results. In contrast to this, the range-free system is a method in which, in the vehicle cabin inside or outside determining process, based on the presence or absence of a response signal for a detection signal between an on-vehicle antenna of which the position is known and the portable device, a relative position (presence/absence) is estimated in the in-vehicle device or the portable device.
In Japanese Patent No. 4483236, a keyless entry device is disclosed which respectively transmits signals from antennas disposed inside and outside the vehicle cabin and determines whether the position of the portable device is inside or outside the vehicle cabin based on whether or not a portable device responds to a signal transmitted from a certain antenna. In other words, for a vehicle cabin inside/outside determination disclosed in Japanese Patent No. 4483236, the range-free system performing a determination based on only the presence/absence of a response from the portable device for a signal transmitted from the in-vehicle device side is employed.
In Japanese Patent No. 4673230, an invention relating to a keyless entry device performing a boundary surface inside or outside determination such as the inside or outside of the vehicle cabin of the portable device, within or beyond a predetermined distance from a door, or the like is disclosed. In Japanese Patent No. 4673230, a method of determining the inside or outside of a boundary surface using the range-based system is disclosed in which the inside or outside determination is performed by measuring a received signal strength of a signal transmitted from an on-vehicle antenna on the portable device side. Particularly, in Japanese Patent No. 4673230, it is disclosed that the boundary surface inside or outside determination is performed with high accuracy also in case where an on-vehicle antenna is configured to be attached to a movable member such as a door mirror, a sheet, or a handle. More specifically, it is disclosed that a degree of similarity with a data group and a parameter in that electric wave strengths that are different inside and outside the boundary surface obtained in advance and an identification code of the on-vehicle antenna are associated with each other, and the degree is derived by calculating a Mahalanobis distance, and the data group and the parameter that are comparison targets are switched in accordance with states before and after the movement of the movable member.
In U.S. Pat. No. 5,165,618, an invention relating to a keyless entry device associated with the invention disclosed in Japanese Patent No. 4673230 is disclosed. In Japanese Patent No. 5165618, particularly, it is disclosed that the calculation is performed using the electric wave strength from an on-vehicle antenna that has failed as zero for deriving the degree of similarity by calculating a Mahalanobis distance between a measured electric wave strength and electric wave strengths obtained in advance which are different inside and outside the boundary surface, even if the on-vehicle antenna has failed, so that the accuracy of the determination of the position of the portable device is maintained as possibly as can.